Measurement devices of the vibration type are also known as Coriolis flowmeters and are used to measure fluid mass flowrates mechanically, and are applied in installations in which the precision of the mass flow is relevant, for example in refineries. Measurement devices of the vibration type can also be used to measure the viscosity and 7 or the density of the fluid flowing through the measurement tube.
DE 103 56 383 A1 discloses a Coriolis mass flowmeter of this generic type which essentially comprises an inlet flange and an outlet flange which produce the connection between the inlet and outlet pipe limbs and external inlet and outlet pipes. A measurement tube connects the inlet and outlet pipe limb, so as to form the shape of a Greek letter Ω. This shape is distinguished by a good oscillation behavior. In addition, embodiments with duplicated, parallel pipe runs are also generally known.
Every Coriolis flowmeter is based on the following physical principle:
An excitation unit produces a periodic harmonic oscillation which is used to excite the measurement tube. The oscillations, which are detected by sensors, at the inlet and outlet points are in the same phase. When flowing through the measurement tube, the fluid mass experiences accelerated oscillation diffractions, which produce a Coriolis force. The originally harmonic oscillation of the measurement tube is therefore influenced by the Coriolis force, which is distributed along the measurement tube, causing a phase shift at the inlet and outlet points. The oscillation phases and oscillation amplitudes at the inlet and outlet points are recorded using sensors, and are passed to an electronic evaluation unit. The magnitude of the phase shift is a measure of the mass flowrate.
Every Coriolis flowmeter is calibrated in order to determine the relationship between the phase shift and the mass flowrate. Calibration means that calibration constants for the measurement instrument are determined under defined conditions, such as a defined temperature, flowrate, etc, and these calibration constants are applied when calculating the mass flowrate from the measured phase shift. The calibration constants are particularly dependent on the pressure within the measurement tube. Pressure within the measurement tube may be a static pressure, that means a pressure under the static condition that the fluid within the measurement tube has zero flowrate. Pressure within the measurement tube may, however, also mean the dynamic pressure when the fluid is flowing with a flowrate other that zero.
The magnitude of the phase shift is dependent on the spring stiffness and the mass of the pipe run. The spring stiffness is in turn dependent on the pipe geometry; in particular the wall thickness, as well as the material characteristics. As long as the material characteristics and the pipe geometry remain unchanged, the calibration that is carried out remains valid, that is to say accurate measured values are achieved. However, if the geometry of the measurement tube—for example the wall thickness—decreases as a result of wear or increases as a result of deposits, incorrect measured values are emitted. A further disadvantage is the increase in the material fatigue over the course of the life, which can become critical, in particular as a result of a reduction in the wall thickness, and in the extreme causes component failures in the form of fatigue fractures. Fatigue fractures such as these are caused by the measurement medium being pressurized, resulting in a corresponding stress on the measurement tube wall.
In order to solve this problem, proposals have been made in the said prior art for the excitation unit to emit a single oscillation pulse to the measurement tube, whose oscillation response is detected by sensors, from which the downstream evaluation unit—in addition to determining the mass flowrate—also calculates the instantaneous damping constant of the measurement tube, and compares this with a stored original damping constant for the measurement tube when new, with the comparison result being used to provide signal-processing compensation for the measurement accuracy.
This has the disadvantage that this measure allows the instantaneous wear state of the measurement tube to be determined only at periodic time intervals, for which purpose normal measurement operation of the flowmeter must be interrupted. This leads to correspondingly increased maintenance effort.